Method of tin coating ferrous metal articles and bath therefor



Patented Feb. 27, 1951 METHOD OF TIN COATlNG FERROUS METAL ARTICLES ANDBATH THEREFOR Arch W. Harris, Euclid, Ulric, assignor to The AmericanSteel and Wire Company of New Jersey, a corporation of New Jersey NoDrawing. Application June 20, 1945,

Serial No. 600,637

This invention relates to improvements in the art of coating metal withtin, and more particularly to the coating of ferrous metal wire with tinfor the purpose of providing a lubri cant for the drawing thereof.

My invention comprises applying a tin coating on ferrous metal articlessuch, for example, as iron or steel wire, by a simple immersion in anaqueous bath of a tin salt. In practicing the improved process, theferrous metal articles which are to be tin coated are first providedwith a coating of copper by immersion in an aqueous solution of a coppersalt, such as copper sulphate, acidified with sulphuric acid. Thismethod of coating ferrous articles with copper by simple immersion isWell known in the art, and processes have been proposed heretofore forsimilarly coating ferrous metals with tin by immersion in atin-containing solution. In all such prior processes, a prolongedimmersion time is required for the formation of the coating of tin, i.e., several hours in the usual operation.

My invention materially reduces the required immersion time to the orderof minutes instead of hours, and assures a uniform coating of tin overthe entire article being treated,

In a preferred practice, an initial coating of copper is applied in themanner indicated above. After the copper coating has been applied, theresulting copper-coated articles are immersed in a tin coating bath,which preferably is composed of a mixture of stannous chloride orstannous sulphate and zinc chloride, the mixture of tin salt and zincchloride being dissolved in a suitable quantity of water as will bepointed out in greater detail hereinafter. A quantity of metallic tin isplaced in the solution for the purpose of preventing oxidation of thestannous salt and to maintain the soluble tin content of the bath.Feathered metallic tin is preferred, because of the large area ofsurface presented thereby.

The following specific example illustrates a preferred practice of theinvention, but is intended to be illustrative only in character, and notto limit the process in the various aspects thereof to the specificconditions described.

In carrying out the improved process, I provide a bath composed ofapproximately 0.75 pound of stannous chloride and zinc chloride each,per

gallon of water, and 0.2 pound per gallon of feathered tin. The tinremains in the bath in metallic form for a purpose to be explainedshortly. After being coated with copper as explained above, the ferrousarticles are immersed filaims. (Cl. 11750) I in the bath just described,which is kept heated approximately to the boiling point. This solutionwill deposit about three grams of tin coating per kilogram of 0.637 inchhigh carbon steel wire in approximately ten minutes at boilingtemperatures. The copper coating on the article is replaced by tin inthe approximate ratio of two atoms of copper by one of tin. Instead ofthe stannous chloride, stannous sulphate may be em-v ployed. Thereplaced copper forms the chloride or sulphate, depending upon whetherthe tin salt employed is stannous chloride or stannous sulphate.Deposition of tin on the article tends to deplete the'tin content of thesolution but the depletion isiimmediately made up by solution of thefeathered tin at the bottom of the bath. The concentration of the baththus remains constant and the copper salts formed as a result ofreplacement of the copper coating by tin crystallize out of solution andsettle to the bottom of the bath.

Heating the bath facilitates deposition of the tin, and it is preferredto operate at'about the boiling point of the solution, although lowertemperatures may suffice for certain purposes. Any water-soluble tinsalt may be em loyed, the stannous chloride and stannous sulphate beingthe most readily available. The function of the zinc chloride is notunderstood, but I have observed that its presence is necessary in orderto give the desired speed in the deposition of the tin.

I am aware that it has been proposed to coat ferrous articles by dippingthem in a fused bath of tin salts, such as those mentioned herein. Theuse of such bath, however, is characterized by very definitedisadvantages. Thus, it is difficult to maintain the establishedphysical properties of the wire after it has passed through a moltensalt bath, which must be maintained at 600 F., whereas no difficulty atall is encountered in maintaining the rare-established physicalproperties of a cold-drawn wire if the same be coated with tin by usingthe aqueous solution described above, even when it is heated nearly toits boiling point.

A further disadvantage is that the tin coating obtained by using thefused salt bath is the same type obtained by using a bath of molten tin,that is to say, the coating is many times heavier than needed to serveas a lubricant in drawing the wire, the tin forming an alloy with thesteel base. In using the fused salt bath, moreover, it is necessary, asin the case of hot-dip tinning, to wipe the coated wire in order toobtain a smooth coating which is necessary to facilitate drawing. Myprocess results in a tin coating which is smooth enough without wipingto serve satisfactorily as a lubricant when drawing the wire. In drawingwire with a tin coating a plied by my invention it has been foundpossible to make twelve drafts on the cone machine producing a musicwire finish.

It will be apparent that various modifications in the proceduredescribed may be made without departing from the spirit of the inventionor the appended claims.

I claim:

1. In a process of coating ferrous articles with tin, the stepsincluding immersing the articles in an acidified aqueous solution of acopper salt, thereby causing a layer of copper to be deposited on thearticle, thereafter immersing the coppercoated articles in a bath of anaqueous solution of a water-soluble stannous salt and zinc chloride.having a quantity of metallic tin therein and holding the articles inthe said bath until the copper coating thereof is substantially entirelyreplaced by tin, the metallic tin in the bath replacing the tin platedout of the bath.

2. A process as defined by claim 1 characterized by said aqueoussolution being composed of approximately 0.75 pound per gallon of watereach of stannous salt and zinc chloride.

3. A process as defined by claim 1 characterized by heating the bath toa temperature approaching the boiling point of the solution.

4. A process as defined by claim 1, wherein the stannous salt isstannous sulphate.

5. A process as defined by claim 1, wherein the stannous salt isstannous chloride.

6. In a process of finish-coating a ferrous product having an initialcoating of copper thereon, the steps including immersing the product ina bath of an aqueous solution of a water-soluble stannous salt and zincchloride, having a quantity of metallic tin therein and continuing theimmersion until the copper coating on the prodnot has been substantiallyreplaced by tin.

7. A coating bath for depositing tin by displacement of a copper coatingon ferrous articles comprising an aqueous solution of substantiallyequal parts of a water-soluble tin salt and zinc chloride and containingmetallic tin.

8. A coating bath as defined by claim '7 characterized by said solutionbeing composed of substantially 0.75 pound per gallon of water each of awater-soluble tin salt and zinc chloride, and containing approximately0.2 pound of metallic tin per gallon.

9. A coating bath as defined by claim 7, wherein the tin salt isstannous sulphate.

10. A coating bath as defined by claim 7, wherein the tin salt isstannous chloride.

ARCH W. HARRIS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 97,417 Lothrop Nov. 30, 18691,256,084 Watkins Feb. 12, 1918 2,144,798 Epperson Jan. 24, 19392,159,510 Pavlish May 23, 1939 2,282,511 Bradley May 12, 1942 2,369,620Sullivan Feb. 13, 1945 FOREIGN PATENTS Number Country Date 240 GreatBritain 1857

1. IN A PROCESS OF COATING FERROUS ARTICLES WITH TIN, THE STEPSINCLUDING IMMERSING THE ARTICLES IN AN ACIDIFIED AQUEOUS SOLUTION OF ACOPPER SALT, THEREBY CAUSING A LAYER OF COPPER TO BE DEPOSITED ON THEARTICLE, THEREAFTER IMMERSING THE COPPERCOATED ARTICLES IN A BATH OF ANAQUEOUS SOLUTION OF A WATER-SOLUBLE STANNOUS SALT AND ZINC CHLORIDE,HAVING A QUANTITY OF METALLIC TIN THEREIN AND HOLDING THE ARTICLES INTHE SAID BATH UNTIL THE COPPER COATING THEREOF IS SUBSTANTIALLY ENTIRELYREPLACED BY TIN, THE METALLIC TIN IN THE BATH REPLACING THE TIN PLATEDOUT OF THE BATH.